Propter Doc published an interesting post today concerning her to-do list, which she characterized as more of a pyramid:

At the top there is a task, with some kind of importance. Not necessarily the most important thing to me, but someone has told me that I must do that before all other tasks. So then it becomes the apex of the pyramid. Immediately below it, on the next level, are the couple of that I think are important or urgent. The ones that are important to me. The next level is populated by those tasks that are on the horizon, not yet important but still demanding of some attention. Now, the problem starts when that top task becomes difficult for some reason. In this pyramid scheme I'm not allowed to move beyond the top level until the task there is complete.

She recognizes that this is counterproductive, as it keeps her from getting to what is really important for fulfilling her professional goals as an academic scientist. Some thoughts on how to construct a more self-fulfilling to-do system are below the fold.

I forget exactly where I came across this idea, but someone somewhere developed the brilliant idea of thinking of all professional "to-do" tasks along two dimensions: importance and urgency. Importance relates to how important the task is to fulfilling your own professional goals. Urgency relates to how soon the task needs to be achieved in order to have a positive effect on achieving those goals.
If you are an academic scientist, having a finely tuned impressive-looking up-to-date CV is important, but it is not particularly urgent; you can work on updating and improving your CV in free moments here and there, but there is no deadline. If there is a weakly interesting seminar today in an hour, that is very urgent, but it is not important. If you don't show up there in an hour, the seminar comes and goes without you, but missing it is not going to effect your career trajectory.
If you have an R01 grant deadline in a week, getting it written extremely well is both urgent and important. That blog post you've been meaning to write about how cute your cat is when he catches a mouse and rips its guts out in front of you is neither urgent nor important.
The key to exploiting this system is to always prioritize the important shit first--both urgent and non-urgent--and only when those tasks are comfortably under control do you then tackle the non-important shit, urgent first. And you have to allow yourself to be comfortable with a bunch of non-urgent, non-important shit never getting done.
A subsidiary key is to realize that frequently the urgent, but non-important, tasks are those that someone else attempts to impose on you. They may be important to someone else, but not to you.
Hapless grad-student: "Dr. Post-doc, I can't figure out how to use this machine, my samples are degrading, and my advisor is going to strangle me if I don't get this analysis done today. Show me how to use this machine, now!"
This is certainly urgent--the samples are degrading--but it is only important to the grad student, not to you. Remember: "Your failure to plan ahead is not my emergency."
As a grad-student or post-doc, performing experiments and analyzing data is always urgent and important. As a faculty member, meeting grant deadlines and submitting manuscripts are urgent and important. These tasks should always be at the top of your priority list. Nothing else--even if it is both important and urgent for someone else--supercedes them.
(If anyone knows who devised this two-dimensional "importance/urgency" system and where it appears, please let me know in the comments, so I can properly attribute credit.)

---'This is certainly urgent--the samples are degrading--but it is only important to the grad student, not to you. Remember: "Your failure to plan ahead is not my emergency."'---
This is why I enjoy reading your stuff. You wouldn't happen to be certain Dr. House, from the Princeton-Plainsboro Teaching Hospital, would you?

I like this logic a great deal. The difference between urgent and important is one I have been trying to pin down for the last year or so. It is also very difficult for early career scientists to set priorities like this because of the demands other people put on them. For example I struggled a week before a job interview with time to prepare the job talk because a grad student was making demands on me almost word for word like those you mention above. Even after arranging for the student to make 'appointments' for assistance, student was abusing the situation. I wish that I'd had a line like 'your failure....emergency' and I would have used it because it would have saved me the 100 or so words I had to use to convey my thoughts.
But thanks, this clarifies things quite a bit.

I read it in Steven Covey's "7 habits of highly effective people". I know a lot of people read that book, but I have seen similar ideas mentioned elsewhere.
I guess the key is self-discipline about how you use your time, which no one really teaches you at Uni.

I also read about this urgent/important scheme in First Things First. It makes a great deal of sense. I unfortunately spend too much time in the not urgent and not important quadrant. I'm working on it.